1. Field of the Invention
The present invention relates to a power supply device comprising a rechargeable battery for supplying power to a load of a motor, a lamp or the like.
2. Description of the Related Art
In a power supply device comprising a rechargeable battery for supplying power to a load of a motor, a lamp or the like, it is necessary to prevent overcharging of the rechargeable battery so as to prevent overheating and explosion of the battery. Thus, conventionally, a microcomputer in many such power supply devices is provided with a function to detect a residual capacity of the rechargeable battery (residual capacity detection function) so as to stop (end) charging the rechargeable battery when the microcomputer detects that the residual capacity has become 100% (full charge). However, in the full charge detection method using the residual capacity detection function, it is required to continuously operate the microcomputer to control at least the amount of charged current to the rechargeable battery and the amount of discharged current from the rechargeable battery to the load, resulting in the problem of power consumption in the microcomputer. Japanese Laid-open Patent Publications No. 2005-278371 and No. Hei 7-143687 disclose power supply devices of this kind with a residual capacity detection function.
On the other hand, a nickel metal hydride rechargeable battery has a characteristic such that the moment when it reaches a state of full charge is a moment when the value of the battery voltage becomes slightly lower than a peak value after (reaching and) passing the peak value. More specifically, the nickel metal hydride rechargeable battery has a charge characteristic (hereafter referred to as a first charge characteristic) that if the charging is started when the nickel metal hydride rechargeable battery has a residual capacity less than a certain threshold value, it reaches a state of full charge when the value of the battery voltage has become slightly lower than the peak value after the value of the battery voltage passes the peak value. Thus, there has been an increase in the number of power supply devices, comprising a nickel metal hydride rechargeable battery and using the first charge characteristic, that determine that the battery has reached a state of full charge when the value of the battery voltage has become slightly lower than the peak value after passing the peak value, so as to stop charging the nickel metal hydride rechargeable battery. Such power supply devices, in contrast to the power supply devices with the residual capacity detection function described above, do not make it necessary to continuously operate the microcomputer to control the amount of charged current and the amount of discharged current, making it possible to reduce power consumption.